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 A131960 a(n) = A000043(n) * A000668(n). 1
 6, 21, 155, 889, 106483, 2228207, 9961453, 66571993057, 140656423562035331011, 55088331748199422233011027879, 17361742620725829882898847100829589, 21607930299479592429924287571917281427329 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS Note that a(3)=155 and a(4)=889 also belong to A119691. Old name was: Composite numbers such that the first factor is the relevant exponent of Mersenne prime (the second factor). a(n) is the smallest k > 0 such that A000668(n)^2 divides 2^k-1. Then lpf(2^k-1) = A000668(n), where lpf(m) = A020639(m). - Thomas Ordowski, Feb 03 2019 LINKS FORMULA a(n) = A000043(n)*A000668(n). - R. J. Mathar, Oct 18 2007 a(n) = A002326((A000668(n)^2 - 1)/2). - Thomas Ordowski, Feb 03 2019 PROG (PARI) expm = [2, 3, 5, 7, 13, 17, 19, 31, 61, 89, 107, 127]; vector(#expm, i, expm[i]*(2^expm[i]-1)) \\ where expm comes from A000043; Michel Marcus, Feb 07 2019 CROSSREFS Cf. A000043, A000668, A002326, A020639, A119691. Sequence in context: A304264 A210443 A179768 * A244299 A143049 A213680 Adjacent sequences:  A131957 A131958 A131959 * A131961 A131962 A131963 KEYWORD nonn,less AUTHOR A.K. Devaraj, Aug 02 2007, Aug 06 2007 EXTENSIONS Terms corrected, edited, and new name by Michel Marcus, Apr 30 2013 a(12) from Michel Marcus, Feb 07 2019 STATUS approved

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Last modified June 27 16:27 EDT 2022. Contains 354896 sequences. (Running on oeis4.)